Conventional pixel radiation sensors are often based on a hybrid approach in which an electronic circuit is bump bonded to a pixel sensor.
There are a number of types of conventional semiconductor imagers and sensors. One class is based on a hybrid pixel sensor arrangement for two-dimensional single particle detection, or single photon detection. Another class uses monolithic active pixel sensors (APS) that are solid state imagers that provide, for each pixel, radiation-sensing, charge-to-voltage conversion, and a reset function.
The hybrid pixel sensor arrangement is mainly used for IR focal planes, Silicon Pixel arrays for single particle detection, X-ray detection and medical imaging. The hybrid pixel sensor permits independent optimisation of the radiation detector characteristics and the pixel readout electronics because they are fabricated on two separate substrates with two different processes. However, this type of pixel sensor has a limit to the minimum achievable pixel dimensions due to the bump bonding technique. So far 50 μm×50 μm has been achieved, but it is expensive and complex to fabricate. Moreover, the hybrid pixel sensor has an input capacitance (100 fF to 200 fF) sufficiently high to limit the operation and noise performance.
Monolithic APS devices are mainly used for visible light imaging together with CCD imagers, but have also been applied for single particle detection. Known monolithic APS devices employ a floating diffusion as a pixel sensor in the form of an n-diffusion/n-well in p-doped silicon substrate, a photo-gate, or a PIN diode formed in amorphous Si:H deposited above the integrated circuit. In these devices, the pixel signal current is integrated using the input capacitance during an integrating time period of a few milliseconds. The integrated current is read out by a source follower MOSFET transistor F1 as shown in FIG. 1 (prior art). Pixel select transistor F3 switches the output of the pixel to a common load F4. The floating node which comprises the junction of the gate of the source follower MOSFET transistor F1, the pixel sensor and the drain of F2, is sequentially reset by a reset MOSFET transistor F2. This has the disadvantage of generating kTC or reset noise far above the intrinsic electronic noise of the amplifier stage. Furthermore, the device shown in FIG. 1 is not capable of discriminating between incident quanta (hits) during the integration period.
For single charged particle detection, the conventional monolithic APS uses, as the sensor element, an 8–12 ohm epitaxial layer of the silicon wafer used in standard commercial CMOS technologies, the layer being a few microns thick. The charge signal collected is, for example, of the order of 80 e− for a minimum ionising charged particle traversing a 1 μm thick silicon layer. A major drawback of the conventional bulk silicon sensor is that charge collection is achieved by thermal diffusion of carriers. This intrinsically limits carrier velocity and thus charge collection is slow> Charge collection is also spread over adjacent pixels and not complete.
For single photon detection using an integrated APS with an avalanche gain of, for example, 50, the collected charge per photon may be 50 e−. For such very low signal levels, conventional APS architecture is only marginally usable, if at all, as the signal-to-noise ratio required to detect one visible photon, one X-ray or one charged particle is desirably at least 10 to minimise background noise. This requires a noise floor below 5 e− rms, which cannot be achieved by the conventional APS integrating architectures. These architectures have a conversion gain in the order of 20 μV/e− and a reset noise level of greater than 10 e− rms.
Moreover, the integrating APS architecture of conventional devices cannot measure the timing of particle events, and cannot digitally count each incoming charged particle or X-ray or visible photon. Conventional circuit architectures for hybrid pixel radiation sensors are generally too large, typically, at best 50 μm×50 μm, and consume too much power, for example 30 to 50 μW, and are consequently not usable for monolithic integration of high density pixel sensors with quantum detection capability. The applicant is not aware of circuitry able to process the very low signals required for Single Particle/Photon Detection and imaging (SPD) in monolithic integrated circuits.
The present invention aims to substantially overcome or ameliorate one or more of the aforementioned problems.
In particular, embodiments of the present invention address problems of monolithic integration of active silicon pixels in commercial deep submicron CMOS technologies. Embodiments aim to achieve single particle detection, spatial localisation of single charged particle tracks and single photon detection in contrast to conventional APS designs which integrate the sensor signal current over a certain integrating time period.